There’s an updated version of this post at https://www.lightroomqueen.com/disaster-strikes-corrupted-catalog/
As we said in the last post, Lightroom’s integral backup simply takes a copy of your catalog and places it in dated subfolder, so restoring is a simple matter of opening that backup file.
However let’s exercise a little more caution… you wouldn’t want to risk damaging your last backup, so first, you’ll want to duplicate it and put it back in the right place.
Let’s assume your main catalog is stored at My DocumentsMy PicturesLightroomLightroom 2 Catalog.lrcat, and your existing catalog has somehow become corrupted, or you’ve deleted photos accidentally.
By default your backups would be stored at My DocumentsMy PicturesLightroomBackupsdated foldersLightroom 2 Catalog.lrcat.
First rename that main catalog, rather than deleting it, just in case you want to go back to it. We’ll call it ‘Old Catalog.lrcat’ for the moment.
Now go into your last dated backup folder and select the Lightroom 2 Catalog.lrcat file (or whatever your catalog is called). Copy it (Ctrl-C for Windows, Cmd-C for Mac) and go back to your main catalog folder where your catalog usually lives. Paste your copy of the backup catalog (Ctrl-V for Windows, Cmd-V for Mac) into that main catalog folder. Doing that copy/paste has left your backup catalog exactly where it was, but created a copy in the normal location.
Double click on that catalog to open in Lightroom. If it works correctly, you can now delete ‘Old Catalog.lrcat’, and carry on working.